The present invention relates to a novel turbine generator which is cooled inside with water and comprises stator coils made of a copper group material; and, the invention relates to an operating method of the turbine generator, and a nuclear power plant and an operating method of the nuclear power plant.
In general, with an increase in the capacities of electric machines, cooling of the electric machines becomes important in order to secure their high level of performance and make it possible to reduce their size. As disclosed in Japanese Utility Model No.58-40782 and Japanese Utility Model No. 59-8449, most of the turbine generators for a large scaled nuclear or thermal plant employ a water cooling method in which hydrogen gas is contained inside the machine and water is caused to flow inside the stator coil for cooling hollow wires. The stator coil is composed of a hollow wire made of a material having a major constituent of copper, and the surface is covered with an insulation layer, and the cooling water flows through the inside of the wire. Further, the terminal portion of the stator coil is connected to a connection pipe through a wire box with a solder made of a material having a major constituent of copper.
A water cooling system for cooling the stator coil of a turbine generator constructs a piping circuit by pumping water in a water storage tank using a pump, setting the water temperature to a preset value using a cooler, causing the water to pass through a filter, then supplying the water to the generator so as to cause it to pass through the inside of the stator coil, and finally returning the water to the water storage tank. In order to maintain the purity of the cooling water, the system comprises a purity maintaining circuit for supplying part of the water to an ion exchange resin unit and an exhaust pipe for discharging hydrogen gas accumulated in the water storage tank to the outside of the building.
In the water cooling system of a nuclear turbine generator, when the stator coil made of a copper group material is corroded during operation, there is a possibility that the temperature of the coil will increase to cause an electric breakdown by corrosion products clogging the filter or by the accumulation of corrosion products inside the hollow wire to reduce the flow rate of the cooling water. Therefore, in regard to the water cooling system of a nuclear turbine generator, Japanese Patent Application Laid-Open No. 9-271156 discloses a method wherein, in metallic parts and a heat exchanger made of the copper group material, a cupric oxide (CuO) film is formed on the surface of the metallic parts to improve the corrosion resistance, and Japanese Patent Application Laid-Open No. 10-262351 discloses a method wherein the stator coil is assembled after preforming a cupric oxide (CuO) film on the surface of the coil through oxidation treatment. However, both of the methods improve the corrosion resistance by oxidizing the surface of the stator coil at the initial period to form the cupric oxide (CuO) film, and do not take into consideration the fact that the pre-formed oxide film is worn during operation to deteriorate the corrosion resistance. On the other hand, Japanese Patent Application Laid-Open No. 9-289750 proposes a method that by supplying a predetermined amount of oxygen into the path of the water cooling system based on a measured result of an amount of dissolved oxygen in the cooling water, it is possible to form and maintain the cupric oxide (CuO) film and to lengthen the lifetime of the stator coil. However, this method has a problem in that a hydrogen explosion may occur in the water storage tank or in the atmosphere discharge pipe of the nuclear turbine generator cooling system depending on the method of introducing oxygen into the cooling water.
The phenomenon that corrosion products produced by corrosion of the copper material of the stator coil clog the filter or accumulate inside the stator coil to reduce the flow rate of the cooling water occurs in a nuclear power plant, but does not occur in a thermal power plant.
An object of the present invention is to provide a turbine generator having a stator coil made of a copper group material which has high corrosion resistance, and a method of operating the turbine generator, and a nuclear power plant having the turbine generator and a method of operating the nuclear power plant.
The present invention is characterized by a turbine generator comprising a water cooling system for cooling the inside of a stator coil with water, wherein the water cooling system comprises a water storage tank for storing said water; pipes connecting the water storage tank and the stator coil, the pipes being provided in an inlet side and in an outlet side; a decarbonated air injection unit connected to the inlet side of the pipe by a pipe or a decarbonated air injection unit connected to the water storage tank; a heating means for heating the water, the stator coil having an oxide film on a surface in contact with the water, the oxide film being formed before starting operation of the generator by high temperature water injected with decarbonated air and heated by the heating means; and a control unit for effecting control so that the decarbonated air is always and continuously injected into the cooling water during operation of the generator.
Further, the present invention is characterized by a turbine generator comprising a water cooling system for cooling the inside of a stator coil with water, wherein the water cooling system comprises a heating tank for heating and storing the water, and for supplying the water to the water storing tank; a filter for removing solid objects in the water before supplying the water to the stator coil; an ion exchange resin unit for removing ions in the water; an oxygen concentration meter for measuring the oxygen content in the water which has passed through the filter, the stator coil having an oxide film on a surface in contact with the water, the oxide film being formed before starting operation of the generator by high temperature water injected with decarbonated air and heated by the heating tank; and a control unit for controlling the decarbonated air to be always and continuously injected into the cooling water in the stator coil and for controlling the amount of decarbonated air injected into the cooling water based on a signal from the oxygen concentration meter during operation of the generator.
Further, the present invention is characterized by a method of operating a turbine generator comprising the steps of, prior to starting operation of the generator, forming an oxide film on a surface of the stator coil in contact with the water by high temperature water injected with decarbonated air; and, during operation of the generator, always and continuously injecting the decarbonated air into the cooling water.
Furthermore, the present invention is characterized by a method of operating a turbine generator which comprises the steps of, prior to starting operation of the generator, forming an oxide film on a surface of said stator coil in contact with the water by high temperature water above 50xc2x0 C. injected with decarbonated air; and, during operation of the generator, circulating the cooling water so as to have a dissolved oxygen concentration of 1 to 4 ppm by always and continuously injecting decarbonated air into the cooling water to continuously maintain the oxide film on the inner surface of the coil. The invention also is directed to a method of operating a turbine generator which comprises the steps of, prior to starting operation of the generator, injecting an inert gas into the cooling water using a means for injection an inert gas into the cooling water, the inert gas injecting means being directly connected to a pipe connected to the stator coil, and removing corrosion products inside the stator coil and the pipe using a filter connected to the pipe; after that, before starting operation of the generator, forming an oxide film on a surface of the stator coil in contact with the water by injecting high temperature water above 50xc2x0 C. with decarbonated air; and, during operation of the generator, circulating the cooling water so as to have a dissolved oxygen concentration of 1 to 4 ppm by always and continuously injecting said decarbonated air into the cooling water to continuously maintain the oxide film on the inner surface of the coil.
The present invention is characterized by a nuclear power plant comprising a nuclear reactor; a steam turbine driven by steam obtained by energy from the nuclear reactor; a turbine generator driven by the steam turbine, the turbine generator having a water cooling system for cooling the inside of a stator coil with water, wherein the water cooling system is similar to the above-mentioned water cooling system.
The present invention is directed to a method of operating a nuclear power plant comprising a steam turbine driven by steam obtained by energy from a nuclear reactor; and a turbine generator driven by the steam turbine, the turbine generator having a water cooling system for cooling the inside of a stator coil with water, wherein an oxide film is formed and maintained on a surface of the stator coil by injecting decarbonated air into the cooling water inside the stator coil before starting and during operation of the generator.
That is, the present invention is directed to a system which completely suppresses corrosion when a cupric oxide (CuO) film is formed on the surface of the stator coil in contact with the water and maintains the film, and a method of operating the system. This can be attained by injecting decarbonated air into the piping system connected to the stator coil before and during operation the turbine generator or always and continuously injecting the decarbonated air into the cooling water in the water storage tank during formation of the oxide film to circulate high temperature water, preferably above 50xc2x0 C. and having a concentration of dissolved oxygen in the cooling water of 1 to 4 ppm, in the cooling water piping system. Further, the present invention can be attained by circulating the water in the cooling water piping system and injecting an inert gas such as nitrogen at predetermined periods to remove corrosion products in the cooling piping system, then injecting the decarbonated air, circulating the high temperature water above 50xc2x0 C. and having a concentration of dissolved oxygen in the cooling water of 1 to 4 ppm to form the film, and continuously operating the system. It is preferable that the water used is high in purity. The reason for this is to suppress the occurrence of local corrosion, such as pitting corrosion caused by the treatment. It is preferable that the oxidizing temperature before operation is 50 to 90xc2x0 C., and it is more effective when the temperature is higher.
Although atmospheric air normally contains carbon oxide gas of 0.04 weight %, it is preferable for the present invention that the amount of carbon oxide in the cooling water is less than 100 ppm, preferably less than 50 ppm, further preferably less than 10 ppm, furthermore preferably 2 ppm.
Since the corrosion reaction electrochemically progress, the electron transfer reaction can be isolated, and, accordingly, the corrosion can be suppressed by isolating the copper using a dielectric substance from an environment the copper is exposed to. Since cupric oxide (CuO) is a dielectric substance having a dielectric constant of 18.1 at room temperature, corrosion of the copper can be suppressed and the discharging rate of corrosion product of cuprous oxide (CU2O) can be drastically reduced by covering the surface with cupric oxide (CuO) which is the most suitable for a hard oxide film capable of being in intimate contact with a metallic material.
The present invention is characterized in that, in a turbine generator cooling system of the nuclear power plant, the turbine generator cooling system comprises a measuring system for monitoring the concentration of dissolved oxygen in the cooling water, the measuring system being disposed in the cooling water system; and a control unit for injecting decabonating air into the cooling water when the monitored concentration of dissolved oxygen in the cooling water indicates that the concentration of dissolved oxygen in the cooling water is below 1 ppm.
According, to the present invention, in the turbine generator, particularly in the turbine generator of a nuclear power plant, it is possible for the corrosion of the stator coil to be suppressed by forming and maintaining an oxide film inside the stator coil during operation, such that various kinds of problems caused by the corrosion can be avoided and the lifetime of the stator coil of the turbine generator can be lengthened.